with \(\mathbf {c}^{\prime }=\mathbf {c}+i\Omega \mathbf {\eta }\)  and \(c\)  being the stiffness tensor. The tilde in the divergence operator above indicates the coordinates transformation that arises from the PML. As fused silica is an isotropic material, the stiffness tensor has only two independent coefficients, \(c_{11}=78.5\)  GPa and \(c_{44}=31.2\)  GPa [1]}, as well as the viscosity tensor, \(\eta _{\text{11}}=0.00279\)  Pa\(\cdot \) s and \(\eta _{\text{44}}=0.00104\)  Pa\(\cdot \) s [1]}, [3]}. This equation was solved in COMSOL Multiphysics®Structural Mechanics module using isotropic materials (details in Subsection E) with the following parameters: Young's modulus, \(E=c_{44}(3c_{11}-4c_{44})/(c_{11}-c_{44})=73\)  GPa, Poisson's coefficient, \(\nu _P=(c_{11}-2c_{44})/[2(c_{11}-c_{44})]=0.17\) , bulk viscosity coefficient [4]}, \(\eta _{\text{b}}=2\eta _{44}/3+\eta _{11}=0.00348\)  Pa\(\cdot \) s.
